Conventional high voltage power systems in which voltage must be controlled employ a high voltage transformer and filter and a shunt regulator. For direct current bias control, a shunt regulator amplifier leads from the load and is connected to the base of a shunt transistor, which shunts a part of the applied power to bypass the high voltage transformer, depending upon the bias at the base of the shunt transistor. Direct current power transmission also requires the use of a rectifier and filter network. In conventional systems an alternative technique which is especially useful in connection with high frequency alternating current high voltage power supplies is a flyback technique in which a high frequency transformer transmits power through a filter network and a flyback rectifier is connected across the load. A regulator transistor is also required at the primary of the high voltage transformer in this arrangement to alter the power to be applied to the primary of the transformer. Power transistors have only a limited application in regulating the output of high voltage power supplies, however. Above two kilovolts regulating transistors are frequently unstable and short lived. The cost and inconvenience of replacement of such high voltage regulating transistors is a significant disadvantage associated with conventional high voltage power supply regulating systems.
An additional disadvantage of conventional high voltage power supply regulating systems is the characteristic feature of regulating either the power input at the primary of a high voltage transformer, or the power output of the secondary of the transformer. In either case a specific amount of power is derived at the secondary output leads and provided to a load. This means that a separate secondary and power regulating or shunt regulating transistor is required for each load to which power is to be supplied. Power transistors can not be connected in series with the electrical loads because they tend to break down rapidly under high voltages of from two to six kilovolts, such as are employed to power corotrons in xerographic reproduction devices. By employing the light dependent resistors in the present invention, which can accommodate such high voltages, an arrangement is provided for separately regulating the power provided to each load through a series connected device. This means that a plurality of such loads may be fed from the secondary of a single high voltage transformer. By employing a feedback circuit to control the series connected light dependent resistor by impressing a desired low voltage signal to an associated light emitting diode, regulation of a plurality of high voltage direct current power supplies to different circuits using a single high voltage transformer can be effectuated. In such an arrangement, power is fed directly from the rectifier-filter on the secondary windings of the power source to the light dependent resistor before reaching the load. Each light dependent resistor can be regulated separately by a separate feedback amplifier and a dedicated light emitting diode.
It is an object of the present invention to provide a stable, durable current regulating device for a high voltage power supply system. The invention has particular applicability to the supply of power to corotrons in xerographic reproducing devices where the power supply exceeds two kilovolts to each corotron and where different corotrons require independent voltage adjustments.
A conventional form of corona discharge device for use in xerographic reproduction systems is shown generally in U.S. Pat. No. 2,836,725 in which a conductive corona electrode in the form of an elongated wire is connected to a corona generating direct current voltage. The wire is partially surrounded by a conductive shield which is usually electrically grounded. The surface to be charged, called a plate, usually takes the form of a rotatable drum and is spaced from the wire on the side opposite the shield and is mounted on a grounded substrate. A corona discharge current flows partially to the plate or drum and partially to the shield. An alternative form of corotron may be biased in a manner taught in U.S. Pat. No. 2,879,395 wherein an alternating current corona generating potential is applied to the conductive wire electrode and a direct current potential is applied to the conductive shield partially surrounding the electrode to regulate the flow of ions from the electrode to the plate. Other biasing arrangements are known in the prior art and will not be discussed in great detail herein.
A further object of the invention is to provide a means by which current flow to the plate of a corotron can be controlled indirectly by directly controlling the current flow between the corotron wire and the corotron shield. Frequently the corotron plate is physically grounded by mechanical means. To directly derive current flow in the plate for use in feedback control would require electrical insulation of the mechanical elements, thus adding to the physical complexity of the corotron device as well as introducing the possibility of electrical malfunction into an area where practically no such possiblity presently exists.
A further object of the invention is to eliminate the requirement for power transistors for power regulation of a high voltage power supply. Power transistors operated at voltages in excess of two kilovolts are often unstable and lack durability, and hence require frequent replacement. The light dependent resistors employed in accordance with the present invention, on the other hand, have a 10 kilovolt rating and can dissipate 20 watts of power. They are not subject to overloading when operated within their rated limits, and hence are much more reliable than are power transistors for high voltage power regulation.
The invention may be explained with greater precision and clarity by reference to the accompanying drawings.